This invention relates to power monitoring systems for controlling the output of a laser, and more particularly to a vertical cavity surface emitting laser (VCSEL) power monitoring system using plastic encapsulation and index of refraction discontinuities for reflecting a portion of the radiated light from the VCSEL onto a photodetector.
VCSELs are semiconductor lasers that emit light from the chip surface, as opposed to conventional edge emitting semiconductor lasers that emit light from the cleaved edge of the device. In fiber optic systems and certain other applications, an optical subassembly for coupling the laser beam to the fiber includes a metal package and a ball lens. The metal package forms a hermetic seal for the laser and a photodetector, and further includes a cap having an aperture covered by a window that is substantially parallel to the output facet of the laser. The photodetector is positioned to monitor a portion of the radiated light reflected from the window and the lens. The output current of the photodetector, which is proportional to the amount of light incident upon it, is fed back as input to the drive circuitry for adjusting the drive current to the laser in order to maintain a consistent laser output over temperature.
A significant disadvantage of conventional, hermetic packages is high cost. A plastic encapsulated package for the VCSEL and photodetector provides cost savings in both materials and manufacturing processes by utilizing industry standard optoelectronic molding techniques. Another disadvantage of the conventional package is that depending on the beam qualities of the laser, a varying fraction of the radiated light may be incident on the photodetector, which results in inaccurate monitoring. For low divergence beams, only the back reflection from the lens reaches the photodetector while the beam from the window falls back on the laser itself. As output increases with increasing drive current, the beam diverges, causing the back reflection from the window to reach the diode and the back reflection from the lens to walk off of the photodetector. This spatial filtering results in inconsistent monitoring and feedback over the specified range of drive currents and temperatures. The detrimental effect is increased by modal variations in the laser output which further cause the drive circuit to improperly compensate for variations in the photocurrent that are not representative of the laser output. Those skilled in the art would desire a VCSEL power monitoring system that provides consistent power monitoring and feedback at substantially all currents and temperatures within the normal operating range of the VCSEL.
In response to the drawbacks of the prior art, U.S. Pat. No. 5,809,050 (the ""050 patent) discloses a number of different power monitoring systems for VCSELs that purport to generate a light beam having a controlled intensity. The disclosed systems include packages housing a VCSEL, a photodiode and a beamsplitter for dividing the radiated beam into a fraction and a remainder, the remainder being the light beam output from the package. The beam splitter operates by diffraction, scattering, or transmission to direct a fraction of the radiated light beam towards the photodiode. While addressing some of the disadvantages of the prior art, the package disclosed in the ""050 patent is unnecessarily complex as it requires precise alignment of the photodetector within the package to receive the fraction of the radiated light. In the only embodiment disclosed in the ""050 patent that involves partial reflection of the radiated light, the photodiode is mounted perpendicular to the laser with an angled beamsplitter mounted in the optical path at a 45 degree angle between the devices. The package disclosed in the ""050 patent, however, requires tight manufacturing tolerances. First, the photodiode must be aligned with respect to the laser, and then the beamsplitter must be placed at the correct angle and oriented with respect to both the laser and photodiode to properly reflect a fraction of the radiated beam on to the photodiode.
In addition, U.S. Pat. No. 5,812,582 also discloses a power monitoring system utilizing a hermetic package with a window to reflect a portion of the light emitted from the VCSEL to a photodiode mounted in the package. However, both the ""050 and ""582 patents have the disadvantages that they require costly materials and manufacturing processes. In addition, the power monitoring systems disclosed in the ""050 and ""582 patents may result in inaccurate monitoring due to varying divergence angles of the beam emitted from the VCSEL.
There is therefore provided according to the present invention, a plastic encapsulated VCSEL power monitoring system that is relatively easy to manufacture and provides consistent power monitoring and feedback. The system includes a package containing a plastic encapsulated VCSEL, a photodetector, and a tilted beamsplitter, also referred to as a window, for reflecting a portion of the light from the VCSEL toward the photodetector.
In a presently preferred embodiment the VCSEL and photodetector are mounted adjacent to each other on a metal leadframe. The VCSEL, photodetector and leadframe are encapsulated in an optoelectronic plastic molding material. A tilted window, with a partially reflective coating on one side, is attached to the top of the plastic molding material using an epoxy with a refractive index that substantially matches the molding material. In this embodiment, the plastic encapsulated VCSEL and photodetector can be manufactured at low cost using standard molding techniques without special care given to the optical quality of the top surface of the plastic molding. Minor optical imperfections in this top surface of the plastic molding can be effectively eliminated by the use of the index matched epoxy which attaches the window to the plastic molding. The high optical quality of the window and partially reflective coating provides an excellent surface to reflect a portion of the optical beam back to the photodetector. Also, the tilted window substantially reduces inconsistent power monitoring due to beam divergence and walk-off. The degree of tilt of the window is chosen to capture a certain percent of the transmitted light, which is representative of the entire beam, to enable consistent feedback to the drive circuitry over the specified range of drive currents, taking into account modal distribution and temperature variations.
In another alternate embodiment of the present invention, the plastic encapsulation is adapted to conform to the small form factor (SMFF) package for optical transceivers. Conventional optical transceivers use separate optical subassemblies (OSAs) to couple the light from the source to a fiber (TX), and focus the incoming optical signal from the fiber to a detector (RX). The separate housing for the transmitter and the receiver OSAs in the conventional transceiver results in a bulkier module that requires additional space. This is not in line with the needs of the rapidly growing fiber optics industry whose demands for faster communications call for increasing the number of optical interconnects, which means reducing their size. To address industry needs, the SMFF package has evolved for transceivers in which the maximum distance between the optical axes of the transmitter and the receiver optics may be as close as 750 microns. The present invention includes a plastic SMFF package having a tilted window in the transmitter portion for realizing the previously described benefits on power monitoring.